Filtration media for filtering particulate material from gas streams

ABSTRACT

A composite fabric formed by depositing a web of nanofibers electroblown from a first polymer onto a first support web comprising fibers of larger average diameter than the nanofibers spun from a compatible material, in the absence of an adhesive between the webs, and solvent-bonding the webs together.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to composite nonwoven fabrics suitable foruse as air filtration media, for filtering particulate material fromfluid streams.

2. Description of the Related Art

Filter media typically utilized for HVAC air filters that perform atefficiencies less than 99.97% at a 0.3 micron challenge are eitherglass, cellulose or polymer based. Filters made with media in thisperformance range are typically referred to as “ASHRAE filters” sincethe American Society of Heating, Refrigerating and Air-ConditioningEngineers writes standards for the performance of filter media in suchapplications. Polymer based filter media are typically spunbond ormeltblown nonwovens that are often electrostatically enhanced to providehigher filtration efficiency at lower pressure drop when compared toglass or cellulose media manufactured by a wet laid paper-makingprocess.

Electrostatically enhanced air filter media and media manufactured bythe wet laid process, more specifically with the use of glass fibers,currently have limitations. Electrostatically treated meltblown filtermedia, as described in U.S. Pat. Nos. 4,874,659 and 4,178,157, performwell initially, but quickly lose filtration efficiency in use due todust loading as the media begin to capture particles and theelectrostatic charge thus becomes insulated. In addition, as theeffective capture of particulates is based on the electrical charge, theperformance of such filters is greatly influenced by air humidity,causing charge dissipation.

Filtration media utilizing microglass fibers and blends containingmicroglass fibers typically contain small diameter glass fibers arrangedin either a woven or nonwoven structure, having substantial resistanceto chemical attack and relatively small pore size. Such glass fibermedia are disclosed in the following U.S. Patents: Smith et al., U.S.Pat. No. 2,797,163; Waggoner, U.S. Pat. No. 3,228,825; Raczek, U.S. Pat.No. 3,240,663; Young et al., U.S. Pat. No. 3,249,491; Bodendorf et al.,U.S. Pat. No. 3,253,978; Adams, U.S. Pat. No. 3,375,155; and Pews etal., U.S. Pat. No. 3,882,135. Microglass fibers and blends containingmicroglass fibers are typically relatively brittle, and thus whenpleated, break resulting in undesirable yield losses. Broken microglassfibers can also be released into the air by filters containingmicroglass fibers, creating a potential health hazard if the microglasswere to be inhaled.

It would be desirable to provide a means for achieving ASHRAE level airfiltration while avoiding the above-listed limitations of knownfiltration media.

SUMMARY OF THE INVENTION

In a first embodiment, the present invention is directed to a compositefabric comprising a web of electroblown polymeric nanofiberssolvent-bonded to a first support web comprising fibers of largeraverage diameter than the nanofibers spun from a material compatiblewith said nanofibers, in the absence of an adhesive between the webs.

Another embodiment of the present invention is directed to a process forforming a composite fabric, comprising electroblowing a web of polymericnanofibers and a solvent therefor onto a moving support web comprisinglarger fibers spun from a material which is compatible with saidnanofiber polymer, and applying a vacuum pressure between about 4 mm H₂Oand 170 mm H₂O to the combined webs to solvent-bond the nanofiber web tothe support web.

Definitions

The term “nanofibers” refers to fibers having average diameters of lessthan 1,000 nanometers.

The term “filter media” or “media” refers to a material or collection ofmaterials through which a particulate-carrying fluid passes, with aconcomitant and at least temporary deposition of the particulatematerial in or on the media.

The term “ASHRAE filter” refers to any filter suitable for use inheating, ventilation and air conditioning systems for filteringparticles from air.

The term “SN structure” refers to a multilayer nonwoven materialcontaining a support or “scrim” (S) layer and a nanofiber (N) layer.

The term “SNS structure” refers to a multilayer nonwoven materialcontaining a nanofiber layer sandwiched between two support layers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a prior art electroblowing apparatus forforming nanofibers suitable for use in the present invention.

FIG. 2 is an illustration of a processing line for production of SNSsolvent-bonded fabrics according to this invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a composite nonwoven fabric suitable for use asa filter medium, comprising at least one nanofiber layer and at leastone scrim layer. The nanofiber layer comprises a collection ofsubstantially continuous organic polymeric nanofibers having diametersless than about 1 μm or 1000 nm. Such filter media can be used infiltration applications for removing particulate material from a fluidstream, in particular, particulate material from a gaseous stream suchas air.

Filtration media suitable for use in air filtration applications,including ASHRAE filtration and vehicle cabin air filtration, can bemade by layering one or more nanofiber layer(s) with a scrim layer toform an SN_(x) structure, or by sandwiching one or more nanofiber layersbetween two scrim layers to form a SN_(x)S structure, where x is atleast one. Each nanofiber layer has a basis weight of at least about 2.5g/m², and the total basis weight of the nanofiber layers is about 25g/m² or more.

In the medium of the invention, the nanofiber layer has a thickness ofless than about 100 μm; advantageously the thickness of the nanofiberlayer is greater than 5 μm and less than 100 μm. The thickness of thenanofiber layer can vary depending on the density of the nanofiberpolymer. The thickness of the nanofiber layer can be reduced withoutsubstantial reduction in efficiency or other filter properties if thesolids volume fraction of the nanofiber layer is increased, such as bycalendering or by collecting the nanofiber layer under high vacuum.Increasing the solidity, at constant layer thickness, reduces pore sizeand increases filtration efficiency.

The nanofiber layer in the present invention may be made in accordancewith the barrier webs disclosed in U.S. Published Patent Application No.2004/0116028 A1, which is incorporated herein by reference.

The nanofiber layer is made up of substantially continuous polymericfibers having diameters less than 1000 nm, advantageously between about100 nm and about 700 nm, or even between about 200 nm and about 650 nm,or even between about 200 nm and about 500 nm, or even between about 300nm and 400 nm. The continuous polymeric fibers of the nanofiber layercan be formed by the electroblowing process disclosed in PCT PatentPublication Number WO 03/080905A (corresponding to U.S. Ser.No.10/477,882, filed Nov. 20, 2002), which is incorporated herein byreference. WO 03/080905A discloses an apparatus and method for producinga nanofiber web, the apparatus essentially as illustrated in FIG. 1. Themethod comprises feeding a stream of polymeric solution comprising apolymer and a solvent from a storage tank 100 to a series of spinningnozzles 104 within a spinneret 102 to which a high voltage is appliedthrough which the polymeric solution is discharged. Meanwhile,compressed air that is optionally heated in air heater 108 is issuedfrom air nozzles 106 disposed in the sides or the periphery of spinningnozzle 104. The air is directed generally downward as a blowing gasstream which envelopes and forwards the newly issued polymeric solutionand aids in the formation of the fibrous web, which is collected on agrounded porous collection belt 110 above a vacuum chamber 114, whichhas vacuum applied from the inlet of air blower 112.

The nanofiber layers deposited by the electroblowing process invariablyhave a significant quantity of the process solvent entrained therein. Inprevious embodiments of the composite fabric forming process, such asdisclosed in U.S. Provisional Application Ser. No. 60/639771, filed Dec.28, 2004 and incorporated by reference herein, in most cases a nanofiberlayer was first deposited, and with the aid of vacuum chamber 114, mostof the entrained processing solvent was removed prior to collecting thenanofiber layer into a roll. Then the nanofiber layer was manuallycombined with a scrim layer by adhesive bonding to form a composite SNor SNS fabric.

It has been discovered that depositing the electroblown nanofibers andprocessing solvent directly onto a scrim layer, especially a scrim layercontaining larger fibers made of a material which is compatible with thenanofiber polymer, permits in situ bonding of the nanofiber layerdirectly to the scrim, without addition of a separate adhesive betweenthe webs.

According to the present invention, “compatible” polymers are thosewhich are freely soluble in the processing solvent, or where the scrimpolymer is at least partially soluble in or swellable with theprocessing solvent, or where the nanofiber and scrim polymers haverelative polarities which are not so dissimilar that the polymers willnot adhere to one another in the absence of a separate adhesive. Thepresent inventors believe that the “solvent-bonding” of the presentinvention occurs due to a range of solvent/polymer interactions,discussed below.

Suitable polymer/solvent combinations for electroblowing are describedin WO 03/080905A, and include polyamide/formic acid. The use of a scrimcontaining a discrete layer of polymer fibers that are compatible withthe electroblown fibers will be optimal in obtaining a goodsolvent-bond. In one embodiment, “compatible” polymers means that thedifferent polymer fibers are made from polymers having substantiallysimilar chemical make-ups, i.e. made from similar combinations ofmonomers. For example, nylon-6 fibers can be successfully solvent-bondedto nylon-6,6 fibers with a suitable electroblowing solvent for eitherone of them. It is not necessary that the chemically similar fiberpolymers have the same molecular weight distributions, nor that theamounts of said monomers are necessarily the same, nor that all of themonomers are identical. The relative solubility of the chemicallysimilar polymers in the processing solvent determines the efficacy ofthe solvent-bonding.

According to the solvent-bonding technique of the present invention, thenanofiber polymer can be chemically different from the scrim fiberpolymer, so long as the scrim fiber polymer is compatible with thenanofiber polymer, such as where the scrim fiber polymer is at leastpartially soluble in or swellable with the electroblowing solvent. Asdescribed above, when both the nanofiber and scrim fibers are made fromthe same polymer, or polymers which are soluble in the same solvent, asolvent-weld bond can be formed between the two layers, when adjacentportions of the polymer fibers co-dissolve in the electroblowingsolvent, which is subsequently removed. Likewise, even if the scrimpolymer is not freely soluble in the processing solvent, so long as itis swellable by said solvent, a suitable solvent-bond can be obtained.Alternatively, even when the scrim fiber material is not soluble orswellable in the electroblowing solvent, sufficient “solvent-bonding”can be obtained due to the tackiness of the solvent-swollen nanofibersadhering to the scrim fibers prior to solvent removal, so long as thetwo polymers have compatible relative polarities, i.e. are not toodissimilar in polarity.

Accordingly, under these circumstances, the scrim fibers can bepolymers, combinations of polymer fibers and natural fibers, such ascellulose, rayon, cotton and the like, or even all natural fibers, solong as the tacky, as-spun nanofibers can successfully adhere to atleast some of the scrim fibers.

For example, polyamide nanofibers electroblown in formic acid can besuccessfully solvent-bonded to a polyester fiber scrim according to thepresent invention, even though polyester is only marginally soluble, ifat all, in formic acid. However, polyamide nanofibers electroblown informic acid do not adequately adhere or bond to fibers in apolypropylene scrim, which is highly non-polar (Comparative Example B,below). Thus, nanofibers formed from relatively polar polymers are not“compatible” with highly non-polar polymer scrim fibers, such aspolyolefins, within the meaning of the present invention. It has beenfound that electroblown polymeric nanofibers can be successfully“solvent-bonded” to scrims containing at least a portion of fibers madefrom natural materials, such as cellulose fibers, rayon fibers, cottonfibers and the like. Such natural fiber materials are “compatible”within the meaning of the present invention.

Examples of suitably compatible combinations of polymers/solventsinclude, but are not limited to polyamides, such as nylon-6, nylon-6,6,nylon-6,10 in formic acid, meta-aramid in DMAc (dimethyl acetamide) andpara-aramid in sulfuric acid, polyesters, such as PET (polyethyleneterephthalate), PBT (polybutylene terephthalate) and PPT (polypropyleneterephthalate) in trifluoroacetic acid/methylene chloride orN-methyl-2-pyrrolidone (NMP), PAN (polyacrylonitrile) in DMF (dimethylformamide) or DMAc, PVA (polyvinylalcohol) in water, polyurethanes inDMAc, and PVdF in DMAc or DMF. There are other suitable solvent choicesfor some polyamides, such as HFIP (1,1,1,3,3,3-hexaflurorisopropanol),which also dissolves PET. Those skilled in the art of polymer solubilitywill appreciate that by matching solubility parameters from references(such as the CRC Handbook of Solubility Parameters and Other CohesionParameters by Allan F M Barton), a range of suitable polymer/solventelectroblowing systems can be matched to scrim materials.

It is also likely that scrim layers having relatively rough surfaceswill provide better bonding to the nanofiber layers than scrim layershaving smooth surfaces, such as smooth bonded scrims.

Thus, according to the present invention, a composite fabric is made bysolvent-bonding the nanofiber layer to the scrim by electroblowing ananofiber layer in combination with the entrained processing solventdirectly onto the substantially continuous scrim layer which issupported by the moving collection belt 110, to form an SN structure(FIG. 1).

When solvent-bonding the SN structures, it has been found thatparticularly robust bonding occurs when the nanofiber/solventcombination is deposited on the support layer over the vacuum chambercollector 114 at vacuum levels greater than about 60 mm H₂O, such as atvacuum levels from about 60 mm H₂O up to about 170 mm H₂O, andadvantageously between about 60 mm H₂O to about 100 mm H₂O.

The composite fabric of the invention can be made by forming a nanofiberlayer onto the scrim in a single pass or by building up the nanofiberlayer to the desired thickness or basis weight using multiple passes,e.g., in an electroblowing process. The electroblowing process allows ananofiber layer of suitable basis weight for use in an air filter mediumto be formed in a single pass because a higher polymer solutionthroughput is possible than previously known in the production ofnanofibers. Depending on the polymer solution flow rate and thecollection belt speed, single nanofiber layers having basis weights ofbetween about 2.5 g/m² and even up to 25 g/m² can be formed in a singlepass. The benefits in efficiency of such a new process are obvious tothe skilled artisan. By forming the nanofiber layer in one passaccording to the present invention, less handling is required, reducingthe opportunity for defects to be introduced in the final filter mediumand enabling the nanofiber layer to be solvent-bonded directly to thescrim layer without interrupting the process flow. Of course, thoseskilled in the art will recognize that under certain circumstances itcan be advantageous to use multiple electroblowing beams to depositmultiple nanofiber layers of at least about 2.5 g/m² in succession, inorder to build-up the total nanofiber layer basis weight to as much asabout 25 g/m² or more. Variations in the spinning conditions to modifythe nanofiber laydown rate, and therefore the basis weight of a singlenanofiber layer, can be made in the collection belt speed, polymersolution flow rate and even by varying the concentration of the polymerin the solution.

In another embodiment of the invention, SNS composite fabrics can beformed and bonded in a continuous operation. In this embodiment (FIG.2), a substantially continuous support or scrim layer 122 is suppliedfrom supply roll 121 onto a moving collection belt 110 and is directedinto spin cell 120 and under one or more electroblowing beams 102 todeposit one or more layers of nanofibers containing solvent onto themoving scrim, under moderate vacuum, to form an SN structure 123. Thevacuum level of vacuum chamber 114 is maintained from about 4 mm H₂O upto about 30 mm H₂O with a vacuum blower 112, so as to aid in collectionof the nanofiber layer, but not so high as to remove significant amountsof the processing solvent. Subsequently, a second scrim 125, which canbe the same or different from the support scrim 122, is supplied fromtop scrim supply roll 124 and directed past a first tensioning roll 118,around consolidation roll 126 and deposited on top of the nanofiberlayer(s) with light compression from the consolidation roll to form themulti-layered SNS composite fabric 127. The light compression acts toensure full contact between the adjacent fabric layers to permit thesolvent remaining in the nanofiber layer to soften and weld the fibersof the various layers together. The compression pressure is preferablymaintained so as to ensure adequate contact and bonding between thelayers, but not so high as to deform the individual fibers or tosignificantly reduce the overall permeability of the fabrics.Subsequently, the solvent-bonded SNS composite fabric 127 is directedover another tensioning roll 118, then over a second vacuum chamber 115,operating at higher vacuum levels to remove the remaining solvent andwound on wind up roll 130.

Those of skill in the art will recognize that the optimum vacuum levelsin both vacuum chamber 114 and vacuum chamber 115 will depend largelyupon the polymer/solvent combinations used in the electroblowing andbonding processes. For example, a more volatile solvent may require lessvacuum in either or both stages to achieve the solvent-bonding andremoval functions disclosed herein.

Advantageously, the scrim layers are spunbond (SB) nonwoven layers, butthe scrim layers can be carded, wet laid, meltblown or otherwise formedand consolidated webs of nonwoven polymeric and/or natural fibers, wovenpolymeric and/or natural fiber fabrics and the like. The scrim layersrequire sufficient stiffness to hold pleats and dead folds. Thestiffness of a single scrim layer is advantageously at least 10 g, asmeasured by a Handle-o-meter instrument, described below. Particularlyhigh stiffness can be achieved by using an acrylic bonded carded or wetlaid scrim comprising coarse staple fibers. The scrim layers may bemulti-layered fabrics, such as laminates of layers of cardedpoly(ethylene terephthalate) (PET) fibers and carded nylon fibers, orother such multi-layer fabrics. Advantageously, the filtration medium ofthe invention has a total Handle-o-meter stiffness of at least 45 g, anda structure of SN_(x)S, in which at least two scrim layers contribute tothe stiffness, and the number of nanofiber layers, x, is at least one.In the case of an SNS structure, the two scrim layers can be the same,or can differ as to basis weight, fiber composition or formationtechnique. For example, the support scrim can be a spunbond polyamidenonwoven web, onto which is deposited a polyamide nanofiber layer, andthe top scrim can be a woven, carded or spunbond layer made of a thirdpolymer, so long as the third polymer is compatible with the nanofiberpolymer. Another advantageous combination of layers is an electroblownpolyamide nanofiber layer solvent-bonded to a wet laid nonwoven scrimmade from PET fibers, cellulose fibers, or even blends of PET andcellulose fibers. Another advantageous combination of layers iselectroblown polyvinyl alcohol fibers bonded to a composite wet laidlayer of PVA and rayon fibers.

The composite fabric of the invention can be fabricated into any desiredfilter format such as cartridges, flat disks, canisters, panels, bagsand pouches. Within such structures, the media can be substantiallypleated, rolled or otherwise positioned on support structures. Thefiltration medium of the invention can be used in virtually anyconventional structure including flat panel filters, oval filters,cartridge filters, spiral wound filter structures and can be used inpleated, Z-filter, V-bank or other geometric configurations involvingthe formation of the medium to useful shapes or profiles. Advantageousgeometries include pleated and cylindrical patterns.

The initial pressure drop (also referred to herein as “pressure drop” or“pressure differential”) of the filter medium is advantageously lessthan about 30 mm H₂O , more advantageously less than about 24 mm H₂O.The pressure drop across a filter increases over time during use, asparticulates plug the filter. Assuming other variables to be heldconstant, the higher the pressure drop across a filter, the shorter thefilter life. A filter typically is determined to be in need ofreplacement when a selected limiting pressure drop across the filter ismet. The limiting pressure drop varies depending on the application.Since this buildup of pressure is a result of dust (or particulate)load, for systems of equal efficiency, a longer life is typicallydirectly associated with higher load capacity. Efficiency is thepropensity of the medium to trap, rather than to pass, particulates. Ingeneral the more efficient filter media are at removing particulatesfrom a fluid flow stream, the more rapidly the filter media willapproach the “lifetime” pressure differential, assuming other variablesto be held constant.

It has been discovered that the solvent-bonded composite fabrics of thepresent invention provide an unusual combination of fluid permeabilityand efficiency as compared both to conventional wet laid microglassmedia for air filtration application and to the adhesive bondedcomposites of U.S. Provisional Application No. 60/639771.

The filter medium of the present invention has an efficiency of at leastabout 20%, meaning that the medium is capable of filtering out at leastabout 20% of particles having a diameter of 0.3 μm in air flowing at aface velocity of 5.33 cm/sec. For use in ASHRAE filters, advantageously,the medium of the invention is capable of filtering out at least about30% and up to about 99.97% of 0.3 μm particles in air flowing at a facevelocity of 5.33 cm/sec.

The higher the air permeability of the filter medium, the lower thepressure drop, therefore the longer the filter life, assuming othervariables are held constant. Advantageously, the Frazier airpermeability of the filter medium of the invention is at least about0.91 m³/min/m², and typically up to about 48 m³/min/m².

The filter medium of the present invention is advantageouslysubstantially electrically neutral and therefore is much less affectedby air humidity as compared with the filters disclosed in U.S. Pat. Nos.4,874,659 and 4,178,157, described above, which owe their performancesto the charges associated therewith. By “substantially electricallyneutral” is meant that the medium does not carry a detectable electricalcharge.

Test Methods

Filtration Efficiency was determined by a Fractional Efficiency FilterTester Model 3160 commercially available from TSI Incorporated (St.Paul, Minn.). The desired particle sizes of the challenge aerosolparticles were entered into the software of the tester, and the desiredfilter flow rate was set. A volumetric airflow rate of 32.4 liters/minand a face velocity of 5.33 cm/sec were used. The test continuedautomatically until the filter was challenged with every selectedparticle size. A report was then printed containing filter efficiencydata for each particle size with pressure drop. Efficiencies reported inthe data below are for 0.3 micrometer particle challenge only.

Pressure Drop was reported by the Fractional Efficiency Filter TesterModel 3160 commercially available from TSI Incorporated (St. Paul,Minn.). The testing conditions are described under the FiltrationEfficiency test method. Pressure drop is reported in mm of water column,also referred to herein as mm H₂O.

Basis weight was determined by ASTM D-3776, which is hereby incorporatedby reference and reported in g/m².

Thickness was determined by ASTM D177-64, which is hereby incorporatedby reference, and is reported in micrometers.

Fiber Diameter was determined as follows. Ten scanning electronmicroscope (SEM) images at 5,000× magnification were taken of eachnanofiber layer sample. The diameter of eleven (11) clearlydistinguishable nanofibers were measured from the photographs andrecorded. Defects were not included (i.e., lumps of nanofibers, polymerdrops, intersections of nanofibers). The average fiber diameter for eachsample was calculated.

Stiffness was measured using a “Handle-o-meter” instrument manufacturedby Thwing Albert Instrument Co. (Philadelphia, Pa.). The Handle-o-metermeasures in grams the resistance that a blade encounters when forcing aspecimen of material into a slot of parallel edges. This is anindication of the stiffness of the material, which has an inverserelationship with the flexibility of the material. The stiffness ismeasured in both the longitudinal direction (machine direction) of thematerial and the transverse direction (cross-machine direction).

Frazier Permeability is a measure of air permeability of porousmaterials and is reported in units of ft³/min/ft². It measures thevolume of air flow through a material at a differential pressure of 0.5inches (12.7 mm) water. An orifice is mounted in a vacuum system torestrict flow of air through sample to a measurable amount. The size ofthe orifice depends on the porosity of the material. Frazierpermeability is measured in units of ft³/min/ft² using a Sherman W.Frazier Co. dual manometer with calibrated orifice, and converted tounits of m³/min/m².

EXAMPLES Example 1

Nanofiber layers were made by electroblowing a solution of nylon-6,6polymer having a density of 1.14 g/cc (available from E. I. du Pont deNemours and Company, Wilmington, Del.) at 24 weight percent in formicacid at 99% purity (available from Kemira Oyj, Helsinki, Finland). Thepolymer and solvent were fed into a solution mix tank, the solutiontransferred into a reservoir and metered through a gear pump to anelectroblowing spin pack having spinning nozzles, as described in PCTPatent Publication No. WO 03/080905. The spin pack was 0.75 meter wideand had 76 spinning nozzles. The pack was at room temperature with thepressure of the solution in the spinning nozzles at 10 bar. Thespinneret was electrically insulated and applied with a voltage of 75kV. Compressed air at a temperature of 44° C. was injected through airnozzles into the spin pack at a rate of 7.5 m³/minute and a pressure of660 mm H₂O. The solution exited the spinning nozzles into air atatmospheric pressure, a relative humidity of 65-70% and a temperature of29° C. The polymer solution throughput of the nanofiber-forming processwas about 2 cm^(3/)min/hole. The fibers formed were laid down 310 mmbelow the exit of the pack onto a porous scrim on top of a porous beltmoving at 5-12 m/minute. A vacuum chamber pulling a vacuum of 100-170 mmH₂O beneath the belt assisted in the laydown of the fibers. A 40 g/m²basis weight spunbond PET nonwoven material (Finon C 3040) obtained fromKolon Company (S. Korea) was used as the scrim. The scrim had astiffness of 35 g in the longitudinal direction and 55 g in thetransverse direction.

The SN structure produced was challenged at various particle sizes forfiltration efficiency and pressure drop using a TSI tester 3160, and theresults are given in Table 1.

Example 2

An SN structure was made as described in Example 1, but at a higherbasis weight of the nanofiber layer. The resulting structure waschallenged at various particle sizes for filtration efficiency andpressure drop, and the results are given in Table 1. TABLE 1 NanofiberNanofiber Pressure Frazier air Ex. diameter basis weight Efficiency Droppermeability No. (nm)* (g/m²) (%) (mm H₂O) (m³/m²/min) 1 341/387 3 69.93.7 37 2 374/362 5 85 6.4 22*first measurement/second measurement

Example 3

A filtration medium having an SN structure was formed as in Example 1 bydepositing a nylon nanofiber layer containing solvent, having a basisweight of about 3 g/m² on a Finon C 3070 spunbond PET scrim having abasis weight of about 70 g/m². The average diameter of the nanofiberswas about 400 nm. The nanofiber web was collected on the scrim under acollector vacuum pressure of 60 mm H₂O to form the composite SN fabric,and the composite fabric was passed through a dryer at 110° C. and at avacuum pressure of 20 mm H₂O. The composite fabric was pleated to form apleated filter medium. The solvent-bonding process resulted in a goodbond of the nanofiber web to the scrim with no delamination and goodabrasion resistance of the nanofiber layer to hand rubbing of thepleated medium. The pressure drop and the efficiency of the filtrationmedium before and after pleating are listed in Table 2.

Example 4

A filtration medium was formed as in Example 3, with the exception thatthe nanofiber layer was collected on the scrim under a collector vacuumpressure of 80 mm H₂O. The composite fabric was pleated to form apleated filter medium. The solvent-bonding process resulted in a goodbond of the nanofiber web to the scrim with no delamination and fairabrasion resistance to hand rubbing of the nanofiber layer of thepleated medium. The pressure drop and efficiency of the filtrationmedium before and after pleating are listed in Table 2.

Comparative Example A

A filtration medium was formed as in Example 3, with the exception thatthe nanofiber layer was collected on the scrim under a collector vacuumpressure of 40 mm H₂O. The composite fabric was pleated to form apleated filter medium. The solvent-bonding process resulted in a goodbond of the nanofiber web to the scrim with no delamination, but thenanofiber layer was easily abraded by hand rubbing of the pleatedmedium. The pressure drop and efficiency of the filtration medium beforeand after pleating are listed in Table 2.

Comparative Example B

A filtration medium was formed as in Example 3, with the exception thatthe nanofiber layer was collected on a spunbond PP scrim. No bonding wasevident.

Example 5

A filtration medium was formed by depositing a nylon nanofiber layercontaining solvent, having a basis weight of about 3 g/m² onto a Finon C3070 spunbond PET scrim from Kolon Company. The nanofiber layer wascollected on the scrim under a vacuum pressure of 4 mm H₂O, and a topscrim of a two-layer carded fabric (HDK Industries, Inc., Greenville,S.C.) was applied. The carded fabric had a layer of carded nylon fibersand a layer of carded PET fibers. The carded nylon fiber layer wasdirected into contact with the nylon nanofiber layer, to form acomposite SNS fabric. The composite SNS fabric was passed through aconsolidation nip to effect solvent-bonding of the nanofiber layer toboth top and bottom scrim layers. Subsequently, the solvent-bondedcomposite was passed through a dryer at a temperature of 90° C. and avacuum pressure of 20 mm H₂O. The composite fabric was pleated to form apleated filter medium. The solvent-bonding process resulted in a goodbond of the nanofiber web to the scrims with no delamination onhandling, and only slight delamination by rubbing two layers of thepleated medium together. The pressure drop and efficiency of thefiltration medium before and after pleating are listed in Table 2.

Example 6

A filtration medium was formed as in Example 5, except that the basisweight of the nanofiber layer was 5 g/m². The composite fabric waspleated to form a pleated filter medium. The solvent-bonding processresulted in a good bond of the nanofiber web to the scrims with nodelamination on handling, and no delamination by rubbing two layers ofthe pleated medium together. The pressure drop and efficiency of thefiltration medium before and after pleating are listed in Table 2.

Example 7

A filtration medium was formed as in Example 5, except that the bottomscrim was a Finon F 5070 spunbond PET scrim and the collector vacuumpressure was 5 mm H₂O. The composite fabric was pleated to form apleated filter medium. The solvent-bonding process resulted in a goodbond of the nanofiber web to the scrims with no delamination onhandling, and only slight delamination by rubbing two layers of thepleated medium together. The pressure drop and efficiency of thefiltration medium before and after pleating are listed in Table 2.

Example 8

A filtration medium was formed as in Example 7, except that thecollector vacuum pressure was 10 mm H₂O. The composite fabric waspleated to form a pleated filter medium. The solvent-bonding processresulted in a good bond of the nanofiber web to the scrims with nodelamination on handling, and no delamination by rubbing two layers ofthe pleated medium together. The pressure drop and efficiency of thefiltration medium before and after pleating are listed in Table 2.

Example 9

A filtration medium was formed as in Example 7, except that thecollector vacuum pressure was 20 mm H₂O. The composite fabric waspleated to form a pleated filter medium. The solvent-bonding processresulted in a good bond of the nanofiber web to the scrims with nodelamination on handling, and only slight delamination by rubbing twolayers of the pleated medium together. The pressure drop and efficiencyof the filtration medium before and after pleating are listed in Table2.

Comparative Example C

A filtration medium was formed by depositing a 3 g/m² layer of polyamidenanofibers on a 30 g/m² spunbond PET base scrim (Finon C 3040) accordingto the present invention, and a preformed 70 g/m² spunbond PET top scrim(Finon C 3040) was adhesive laminated to the nanofiber layer to form anSNS structure. The pressure drop and efficiency of the filtration mediumbefore pleating is listed in Table 2.

Example 10

A filtration medium was formed by depositing a 3 g/m² layer of polyamidenanofibers on a 70 g/m² spunbond PET base scrim (Finon C 3040) and a 30g/m² spunbond PET top scrim (Finon C 3040) was deposited on thenanofiber layer according to the in-line solvent-bonding process of thepresent invention, to form an SNS structure. The pressure drop andefficiency of the filtration medium before pleating is listed in Table2. TABLE 2 Pre-pleat Post-pleat Nanofiber Pressure Pressure Ex. Mediumbasis weight Pre-pleat Drop Post-pleat Drop No. Structure (g/m²)Efficiency (mm H₂O) Efficiency (mm H₂O) 3 SN 3 48.2 2.19 52.4 2.27 4 SN3 40.1 2.08 54.9 2.43 Comp A SN 3 45 1.2 55 2.55 5 SNS 3 41.4 1.57 38.11.68 6 SNS 5 57.4 2.95 48.1 2.4 7 SNS 3 37.5 1.21 35.3 1.68 8 SNS 3 40.31.25 37.8 1.9 9 SNS 3 38.4 1.51 34.8 1.57 Comp C SNS 3 62.3 3.4 10 SNS 365.1 2.8

As discussed above, Comparative Example A which was deposited only 40 mmH₂O vacuum, had insufficient resistance to abrasion, in contrast toExamples 3 and 4 of this invention, which demonstrated fair to goodabrasion resistance. Comparative Example B that utilized a highlynon-poplar PP spunbond scrim could not achieve sufficient bonding of thescrim and nanofiber layers for use as a filtration medium.

Comparative Example C, which was adhesive laminated, demonstratedreduced efficiency and increased pressure drop, as compared to theidentical SNS structure of Example 10, formed according to the in-linesolvent-bonding lamination process of the present invention.

1. A composite fabric comprising a web of electroblown polymericnanofibers solvent-bonded to a first support web comprising fibers oflarger average diameter than the nanofibers spun from a materialcompatible with said nanofibers, in the absence of an adhesive betweenthe webs.
 2. The composite fabric of claim 1, wherein the nanofiberpolymer is selected from the group consisting of polyamides, polyesters,polyurethanes, polyvinylidene fluoride and polyvinyl alcohol, and thesupport web comprises fibers selected from the group consisting ofpolyamides, polyesters, polyurethanes, polyvinylidene fluoride,polyvinyl alcohol, natural fibers and combinations thereof.
 3. Thecomposite fabric of claim 1, wherein the compatible material ispolymeric.
 4. The composite fabric of claim 2, wherein both thenanofibers and the support web fibers are polyamides.
 5. The compositefabric of claim 3, wherein the nanofiber polymer and the polymer fibersof said support web are different polymers.
 6. The composite fabric ofclaim 3, wherein the support web comprises a combination of differentpolymeric fibers.
 7. The composite fabric of claim 6, wherein thesupport web comprises multiple layers of different polymeric fibers. 8.The composite fabric of claim 1, wherein the support web is a wovenfabric or a nonwoven web.
 9. The composite fabric of claim 8, whereinthe support web is a nonwoven web selected from the group consisting ofspunbond fibers, meltblown fibers, carded fibers, wet laid fibers andcombinations thereof.
 10. The composite fabric of claim 1, wherein thenanofiber web comprises polyamide nanofibers having average fiberdiameters between about 200 nm and 500 nm, having a basis weight of atleast about 2.5 g/m², and the support web comprises at least one layerof spunbond fibers.
 11. The composite fabric of claim 1, wherein thenanofiber web comprises polyamide nanofibers having average fiberdiameters between about 200 nm and 500 nm, having a basis weight of atleast about 2.5 g/m², and the support web comprises a bi-layer structureof carded polyamide fibers and carded polyester fibers, and wherein thenanofiber web is solvent-bonded to the carded polyamide fiber layer. 12.The composite fabric of claim 1, further comprising a second support webcomprising fibers of larger average diameter than the nanofibers spunfrom a material compatible with said nanofibers, solvent-bonded to saidnanofiber web opposite the first support web.
 13. The composite fabricof claim 12, wherein said first and second support webs are chemicallythe same.
 14. The composite fabric of claim 2, wherein said nanofibersare polyamide and the support web comprises a blend of PET and naturalfibers.
 15. The composite fabric of claim 14, wherein said naturalfibers are cellulose.
 16. The composite fabric of claim 2, wherein saidnanofibers are polyamide and the support web fibers are all naturalfibers.
 17. The composite fabric of claim 16, wherein said naturalfibers are cellulose.
 18. The composite fabric of claim 2, wherein saidnanofibers are polyvinyl alcohol and said support web comprises a blendof polyvinyl alcohol fibers and rayon fibers.
 19. A process for forminga composite fabric, comprising electroblowing a web of polymericnanofibers and a solvent therefor onto a moving support web comprisinglarger fibers spun from a material which is compatible with saidnanofiber polymer, and applying a vacuum pressure between about 4 mm H₂Oand 170 mm H₂O to the combined webs to solvent-bond the nanofiber web tothe support web.
 20. The process of claim 19, wherein the applied vacuumpressure is between about 60 mm H₂O and 170 mm H₂O, to form asolvent-bonded nanofiber web/support web composite fabric.
 21. Theprocess of claim 19, wherein the applied vacuum pressure is betweenabout 4 mm H₂O and 30 mm H₂O, and further comprising depositing a secondsupport web comprising larger fibers spun from a material which iscompatible with said nanofiber polymer on said nanofiber web, to form asupport web/nanofiber web/support web composite fabric, and passing saidcomposite fabric through a consolidating nip, to solvent-bond thecomposite.
 22. The process of claim 21, further comprising drying thesolvent-bonded composite fabric under vacuum to remove the solvent.